1. Technical Field
The present invention relates to an amplifying circuit that utilizes an operational amplifier having a negative feedback path.
2. Background Art
Conventionally, there have been available amplifying circuits for signal amplification, which have been used for various purposes. For example, with microphone input for a portable phone, an audio processing circuit carries out a process for the input, and a pre-amplifier carries out first-stage amplification on the microphone input.
An electret condenser microphone (ECM) is often used for a microphone, and a pre-amplifier for such a microphone input outputs an electrical signal in accordance with the intensity of a sound, from large sound to small sound, input to the microphone. Such a pre-amplifier has the capability, in terms of a sound level (SPL: sound pressure level), of outputting a full scale output relative to an input of 120 dB SPL, and of having an SN ratio (a signal-to-noise ratio) of 60.5 dB or smaller relative to 94 dB SPL.    Related Art Document: Japanese Patent Laid-open Publication No. 2009-232201
Here, when a signal in excess of 120 dB SPL is input from a microphone (that is, an excessive input level), an output from the pre-amplifier resultantly exceeds the full scale, with the excessive portion being cut off. Such a large volume situation is not very common. However, it is preferable to obtain a pre-amplifier output without distortion even in such a situation. Suppose that an input signal level is 140 dB SPL, for example. This is larger by 20 dB (ten times) than the maximum value of an input level from a conventional microphone. Supposing that the amplitude (Vp-p) of an input signal of 120 dB SPL is 0.44 V, the amplitude of an input signal of 140 dB results in 4.4 V. However, a pre-amplifier comprising an operational amplifier of a voltage input type and using a 1.8 V batter as a power source cannot amplify an input signal having an amplitude of 4.4 V.
In view of the above, the pre-amplifier should be a current amplifying type that receives an input signal from a microphone via a negative input terminal of an operational amplifier. In this case, two operational amplifiers, including one for a normal level and the other for a large level, are provided to obtain an output signal for a normal input and an output signal for a large input via the respective outputs thereof. However, when operational amplifiers are arranged in parallel to distribute signals, an output signal from a normal level operational amplifier can hardly ensure sufficient accuracy. Meanwhile, when operational amplifiers are arranged in series so that an output relative to a large input is obtained at the first stage and the output relative to a large input is amplified to thereby obtain a normal level output, sufficient accuracy can be hardly maintained for the normal level output.